@nogc closures

[Steven Schveighoffer]

On 8/5/18 8:23 AM, vit wrote:
> On Sunday, 5 August 2018 at 10:57:32 UTC, Steven Schveighoffer wrote:
>> On 8/5/18 5:20 AM, vit wrote:
>>> It's possible create something like this without errors?
>>>
>>> void main()@nogc{   //Error: function `app.main` is `@nogc`
>>>                      //  yet allocates closures with the GC
>>>      import std.experimental.all;
>>>
>>>      const int j = 2;
>>>      int i = 0;
>>>      const int[3] tmp = [1, 2, 3];
>>>
>>>      tmp[]
>>>          .filter!((x)scope => x == j)    ///main.__lambda1 closes 
>>> over variable j
>>>          .each!((x)scope => i = x);
>>> }
>>
>> The issue is that filter returns a struct with the lambda in it. (the 
>> FilterRange). So the compiler can't deduce that the lambda never escapes.
>>
>> Note, in your toy example, you can fix it by making j an enum, but I'm 
>> assuming this isn't possible in the real code.
>>
>> -Steve
> 
> Yes, it isn't possible.
> I modify filter a and map from std.algorithm:
> 
> void main()@nogc{
>      import std.experimental.all;
>      const int j = 2;
>      int i = 0;
>      const int[3] tmp = [1, 2, 3];
>      tmp[]
>          .xfilter!((x, j) => x == j)(j)
>              .xmap!((x, j) => x * j)(j)
>          .each!((x) => i = x);
> }
> 
> full code: https://dpaste.dzfl.pl/15664d6da5c8

Cool! Consider writing a PR and see if it's acceptable to put into Phobos.

-Steve